1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic process. A copying machine, a printer, a facsimile machine, and a multifunction device having a combination of these functions are listed as examples of the image forming apparatus.
2. Description of the Related Art
Electrophotographic or electrostatic recording image forming apparatus use a DC contact charging method in which a DC voltage is applied as charging bias to a charging roller, which is a charging unit configured to charge a surface of a photosensitive member serving as an image bearing member, with the goal of being less damaging to the ozone layer, saving power, and being small in size.
Some further aim to reduce the size of a high-voltage unit by using a configuration which keeps the DC voltage applied to the charging roller at a given fixed value. In this case, a change brought to the surface electric potential of the photosensitive member after charging by a change in the film thickness of the photosensitive member or a change in the use environment of the photosensitive member is dealt with by a known method in which an exposure unit exposes other areas than a toner image forming area with light in an intensity weaker than the light intensity used to expose the toner image forming area (Japanese Patent Application Laid-Open No. 2002-296853). Exposing other areas to light in addition to the toner image forming area in this manner is hereinafter referred to as background area exposure.
Background area exposure has also been practiced for other purposes than keeping the voltage at a given fixed value, one of which is to reduce image density differences caused by a transfer memory (Japanese Patent Application Laid-Open No. 2008-8991).
Known methods of background area exposure include one in which the entire area is exposed with a weak light intensity, and one in which the other areas are exposed to light in the same intensity as in the toner image forming area for a shorter length of time than the exposure time of the toner image forming area (Japanese Patent Application Laid-Open No. H08-194355). The former method is hereinafter referred to as analog background area exposure. The latter method is hereinafter referred to as digital background area exposure. Digital background area exposure is effective in the case where exposure with a light intensity is not possible due to the characteristics of a laser element used as the exposure unit, or similar cases.
A known method of predicting the use amount of toner employs a count unit (counter unit: measurement unit) which counts electric signals (video signals) input to a laser driver configured to control a laser element which is provided in the exposure unit. The count unit samples a specific number of video signals in a pre-determined image area and counts a number of ON-state video signals. The coverage rate of a printed image is calculated from the ratio of the number of samples and the number of counts, and provides the basis for the prediction of the toner use amount.
The above method is hereinafter referred to as video count toner use amount predictive detection. Because signals which are actually input to the laser driver are measured directly, the toner use amount can be detected with precision (Japanese Patent No. 4822578).
However, performing the video count toner use amount predictive detection in an image forming apparatus which is equipped with the digital background area exposure function sometimes gives rise to the following problem.
The count unit measures any video signal input to the laser driver no matter what type the signal is. This means that a signal input to the laser driver to expose a non-print area where a toner image is not formed is measured as well. The exposure of the non-print area, however, does not consume toner. As a result of unnecessarily measuring the video signal of the non-print area in a toner use amount prediction through the video count toner use amount predictive detection, the detected toner use amount ends up being larger than the actual amount of toner used. A way to accurately detect video signals of a print area is being sought after for this reason.